2.0 Functional Description
CX28331/CX28332/CX28333
2.3 Receiver
Single/Dual/Triple E3/DS3/STS-1 Line Interface Unit
2.3 Receiver
This section describes the detailed operation of the various blocks in the CX2833i
receiver.
2.3.1 Receive Sensitivity
The receiver recovers data from the coaxial cable that is attenuated due to the
frequency-dependent characteristics of the cable. In addition, the receiver
compensates for the flat loss (across all frequencies) in the various electrical
components and the variation in transmitted signal power.
The CX2833i device is able to recover data that has been attenuated by a
maximum of 900 feet of coax having characteristics and attenuation consistent
with ANSI T1.102-1993, Annex C, Figure C.2. This approximates the
characteristics of AT&T type 734/728 cable; almost the same attenuation
characteristic is achieved by one-half the length of AT&T type 735 cable.
2.3.2 AGC/VGA Block
The Variable Gain Amplifier (VGA) receives the AMI input signal from the
coaxial cable. The VGA supplies flat gain (independent of frequency) to make up
for various flat losses in the transmission channel and for loss at one-half the
symbol rate that cannot be made up by the equalizer. The VGA gain is controlled
by a feedback loop which senses the amplitude of the equalizer output, acting to
servo this amplitude for optimal slicing.
2.3.3 Receive Equalizer
The receive equalizer receives the differential signal from a VGA and acts to
boost the high frequency content of the signal to reduce inter-symbol interference
(ISI) to the point that correct decisions can be made by the slicer with a minimum
of jitter in the recovered data.
The REQH pin is provided to allow lower amounts of equalization (shorter
equivalent cable lengths) for cases where a square-shaped pulse (that does not
meet the DS3/STS-1 standards) is transmitted to the receiver. A square-shaped
input has a much larger high-frequency content and could have overshoots at the
EQ output high enough to cause bit errors. Setting REQH = 0 will lower the gain
and reduce the amount of overshoot.
2-10
Conexant
100985A
CONEXANT [ CONEXANT SYSTEMS, INC ]
